1. Field of the Invention
The present invention relates in general to mechanisms for driving intake and exhaust valves of an internal combustion engine, and more particularly to the valve drive mechanisms of a type which can vary the opening/closing timing and lifting angle of the intake and exhaust valves in accordance with an operating condition of the engine. More specifically, the present invention is concerned with a so-called a variable valve timing and lift mechanism of an internal combustion engine.
2. Description of the Prior Art
In the variable valve timing and lift mechanisms, there is a type in which a cylindrical hollow cam shaft is rotatably disposed on a drive shaft. By making a certain rotation of the cam shaft relative to the drive shaft, the opening/closing timing and lifting angle of the intake and exhaust valves can be changed. Some of the mechanisms of such type are described in Japanese Utility Model First Provisional Publication 57-198306 and Japanese Patent First Provisional Publication 6-185321.
In the mechanisms of these publications, cylindrical hollow cam shafts for respective cylinders are rotatably disposed on a common drive shaft which rotates synchronously with operation of the engine. A circular flange of each cam shaft and a corresponding circular flange of the drive shaft are respectively formed with radially extending guide grooves. An annular disc is put between the circular flanges in such a manner that respective pins provided on opposed faces of the annular disc are slidably engaged with the guide grooves respectively. The annular disc is rotatably held by a control housing which can pivot the annular disc to an eccentric position relative to the cam shaft (or drive shaft). Thus, by changing the eccentric degree of the annular disc, the valve lift characteristic of the corresponding valve of the engine can be varied or adjusted. In the mechanism of the '321 publication, there is employed an eccentric cam for pivoting the control housing in a direction perpendicular to the axis of the cam shaft. That is, the control housing is pivotally supported by a supporting shaft and has a cam receiving circular opening, and the eccentric cam formed on a control shaft is rotatably received in the cam receiving circular opening. By pivoting the control shaft about its axis to a desired angular position by an actuator, the pivoting movement of the control housing is carried out.
In the variable valve timing and lift mechanisms of the above-mentioned type, it is very important to strictly suppress axial displacement of the control housing and to precisely hold the control housing in a desired angular position. In fact, under operation of the engine, a marked force is inherently applied to the cam shaft from a valve spring and thus the annular disc is biased in a direction to be inclined. If the annular disc is actually inclined upon receiving such force, the control housing would be inclined also, which however interrupts a smoothed rotation transmission from the drive shaft to the cam shaft.